Do Seabirds Control Wind Drift during Their Migration across the Strait of Gibraltar? A Study Using Remote Tracking by Radar

Abstract

This study presents data on the directional flying behaviour of the five most abundant seabird species migrating across the Strait of Gibraltar in relation to the wind, as observed from the north coast, based on radar tracking, and identified to species level by visual observations. A total of 318 seabird trajectories were analysed, illustrating the expected east–west or west–east movements in spring and autumn. We hypothesised that the seabirds that cross the Strait channel during their migrations would behave differently with respect to compensation for wind direction, depending on their flight styles, the migratory period, and the prevailing winds. In this regard, our results showed that flapping birds (Razorbill, Puffin, Northern Gannet, and Balearic shearwater) compensated for wind drift independently of the season and the predominant wind direction. This agrees with the theory that suggests that under moderate winds and whenever visual contact with the coastline is present (as in the case of our study), migrants should compensate for wind drift to avoid being drifted towards the coast, off their main direction of flight. However, Cory’s shearwater, an active gliding seabird with long, slender wings, showed an adaptive directional response to wind, allowing it to be drifted in spring when westerly tailwinds were prevalent, but compensated for wind in autumn, when both easterly and westerly winds were similarly frequent. This adaptive flight behaviour allows it to take advantage of the prevailing tailwinds in spring, gaining ground speed and saving energy during its passage through the Strait, while in autumn, more frequent headwind conditions and a more directional migration to the south may favour compensating for wind drift. Our results support the usefulness of bird radar as a remote tool for describing the pattern of animal movements in the marine environment, as well as their behavioural response to atmospheric conditions. These studies are particularly relevant in the current framework of climate change.